hey i know exactly what all you are going through also i have been with my cureent boyfriend for a year now and i have been cheated on 6 or more times i just really do not have the heart to let go..
it really must be one of the most difficult things to do to someone you love and see everyday and obviously it will impact your life greatly them not being there.. i am still with my bf although he has cheated on me 5 or more times i cant let go .. God says always forgive even if the person keeps doing wrong.. i am terrified of the thought of not waking up and him being there and dont like imagining my life without him either way it would be difficult.. my advice to you is if you are very happy then stay but make it very clear how much it upsets you witch can help after my boyfriend heard i got diagnosed with depression he hasnt cheated for the past 6 months.. so go with what makes you happy ..
but remember the saying a cheater always cheats